FF: Works great, even on scanned (image) pdfs. UI is clunky -particularly when resizing text to fit a spot on the form- but this is really useful for filling out forms.
Gem: Allows you to add text, draw (freehand), add signatures, and highlight documents directly in the browser. You can fill out forms, add comments, and save or print the edited PDF. It is designed for basic annotations rather than advanced structural editing of existing text or images.
Key PDF Editing Features in Firefox:
Add Text: Click the text tool (often a "T" icon) to add text anywhere.
Draw & Sign: Use the draw tool to create signatures or handwrite notes.
To use it, open a PDF file in Firefox, and you will see the editing toolbar in the top right corner.